The USDA forecast for corn acres planted in the United States came in well above trade expectations in last week’s June Acreage Report, sending December futures prices tumbling nearly 20 cents in trading last Friday.

The report estimated planted acres from a survey taken in early June, before many farmers had made final decisions on how much they would be able to plant due to flooding and exceptionally wet conditions in the Corn Belt.

The USDA said it plans to resurvey planting acreage in 14 states, one of the largest...